LYCOPODIACEJE. 



409 



late to ovate in form. In the greater number of species the sterile leaves are of two 

 different sizes, the ventral leaves attached to the under or shaded side of the 

 obliquely ascending stem are much larger than the dorsal leaves on the upper side 

 exposed to the light (Fig. 307, A). Both kinds taken together form four longi- 

 tudinal rows {vide infra). On its upper side and near the base each leaf bears a 

 ligule ; the point of attachment of the sporangium is below this on the fertile leaves. 

 The fertile leaves form a quadrangular terminal spike, are uniform in size, and 

 usually of somewhat different form from the sterile ones. This difference is more 

 striking in those species of Lycopodium (Z. clavatum, &c.) which form a terminal 

 spike of sporangia, the leaves of which are 

 usually yellow or at least not green, and 

 broader and shorter than the sterile foliapre- 

 leaves. In other species however (Z. Selago, 

 &c.) the sporangia are seated in the axils of 

 the ordinary foliage-leaves, without forming 

 an externally distinguishable spike. The form 

 of the leaves of Lycopodium, although always 

 simple, is also very various in the different 

 species, in some cases resembling the aci- 

 cular leaves of Conifers, in others broad, 

 and always spreading on all sides. In 

 Psilotum all the leaves are rudimentary, 

 very small, membranous, and scale-like, even 

 the fibro-vascular bundle is wanting in them ; 

 on the underground shoots of these plants, 

 which assume a root-like appearance (true 

 roots are altogether wanting, vide infra), the 

 formation of leaves is still more completely 

 suppressed, and is often only recognizable 

 by the arrangement of the cells near the 

 punctum vegetaiionis. Tmesipteris, which is 

 allied to Psilotum, possesses, on the other 

 hand, large strong leaves. 



The Phylloiaxis is either spiral or de- 

 cussate. In Isoetes the rosettes are arranged 

 spirally, with the divergences '^, jVj 2T' if> 

 the fractions becoming more complicated the 

 larger the number of leaves that are annually 



formed. In Lycopodium the arrangement is also spiral ; and the number of 

 orthostichies is frequently considerable ; but not unfrequently the leaves form in 

 this genus pseudo-whorls in spiral succession, which appear as decussate pairs 

 (Z. complanatum) or as alternating whorls of numerous leaves, as in Z. Selago, 

 where the forked branches begin with pseudo-whorls of three leaves, but then 

 produce others with four and finally five leaves. In the species of Selaginella 

 which have their leaves arranged in four rows, each dorsal and ventral leaf form 

 together a pair, whose median plane, however, does not intersect that of the next 



Fig. 307. — Sela^utelia i>i(£qiialifolia ; A fertile 

 branch (one-half natural size) ; B apex in longitudinal 

 section bearing microspores on the left, macrospores 

 on the right (magnified). 



